1. Field of the Invention
This invention is a system for concentrating suspended solid particles in a fluid by removing essentially pure fluid from the fluid/solid particle slurry.
Filtration is one of a class of mechanical separations which involve the physical removal of a component as a separate phase, such as the separation of a solid from a liquid. Additional mechanical separations are centrifugation, sedimentation, screening and floatation. The other category of separation depends on the tendency of a soluble component to concentrate in one phase or another. Examples of this class of separation are distillation, gas absorption and liquid-liquid extraction.
For filtration, in general, a slurry is forced against a filter medium which is a thin barrier made of natural, synthetic or metallic fibers. The pores of the medium, or septum, are small enough to prevent the passage of nearly all of the solid particles; others impinge on the fibers, consequently a cake builds up on the filter and after the initial deposition the cake itself serves as the barrier. The capacity of the device is governed by the rate of flow of the fluid filtrate through the ever thickening bed formed by the solid particles.
One type of filtration uses membranes to trap the particles. Membrane filtration is usually considered to be divided into two different processes: the filtration of micro-solutes (particles less than 10 angstroms), called reverse osmosis, and the filtration of macro-solutes and suspended particles of a size larger than 10 angstroms, called ultra-filtration.
Ultra-filtration has an advantage over other filtration systems in that ultra-filters do not directly trap excluded particles: hence, the filtration member does not rapidly lose permeability. Ultra-filtration is a process in which a pressurized solution is caused to flow across a membrane surface. The membrane is designed so that water and species smaller in size than the rejected dimensions of the membrane will pass through the membrane, while larger species will be rejected at the membrane surface and pass downstream to be eliminated in a rejection flow.
A problem encountered in the ultra-filtration of large volumes of water is the build-up of rejected species which do not traverse the membrane. Such rejected species, though not trapped within the membrane, accumulate upon the ultra-filtration membrane surface. This phenomenon is called concentration polarization.
Within an ultra-filtration system there is an impressed pressure differential across the membrane. Water and other very small molecules pass through the ultra-filtration membrane. Solutes larger than the rejection size of the membrane travel to the membrane surface but do not traverse the membrane. At the ultra-filtration surface such species accumulate. These species are removed from the surface only by back diffusion into the bulk flow. Since the water flux of ultra-filtration membranes is high, the convective transport rate is initially much higher than the diffusive back transport rate. A concentration of solute, therefore, builds up at the membrane surface until the solutes precipitate and form a gel. The thickness of this gel layer will increase until its hydraulic resistance to water transport reduces the water flux to an equilibrium value. At equilibrium the convection transport equals the diffusive transport and ultra-filtration is inhibited. Once concentration polarization is in control increasing the pressure of the stream will not increase the flux since the higher pressure will cause a thicker layer of gel and hence greater resistance.
Ultra-filtration purification systems present some additional design problems. If the ultra-filtration membranes are allowed to dry out, they irreversibly consolidate and lose their permeability. Thus, they must be maintained in a wet state at all times. Ultra-filtration membranes are also extremely thin, and large membrane areas must be maintained for any commercial scale operation. The probability, therefore, of a leak developing somewhere in a large commercial system is significant.
Another distinct characteristic of conventional ultra-filtration systems is that the transfer through the membrane filter is improved when a turbulent flow is ensured. Turbulent solution transport that passes normally as well as transversely to the flow direction carries the solution vigorously to the membrane on one hand and also ensures removal of retained components on the other hand. Retained components will inhibit the permeation of the membrane either due to concentration build-up or deposits on the membrane.
For example, most radioactive, toxic and hazardous waste streams contain both suspended solids and dissolved solids. Generally the liquid contains very small amounts of the suspended solids. In order to treat the liquid to remove the dissolved solids, however, it is necessary to first remove the particulates. The filtration equipment currently used for this purpose consists of cartridge filters, belt-type filters, pre-coat filters or granular filters. In all cases the removed solids and the filters will be contaminated and must be specially handled to prevent the spread of contamination. Also, depending on the amount of material to be treated, more than one filter may be needed.
A device and method is therefore desired that would allow the removal of fluid from a fluid/solid particle slurry without the disadvantages of filters in general and ultra-filtration systems in particular. For any filtering system the underlying problem is the removal of accumulated solids on the filter surface that impede the flow of fluid therethrough and require that operation of the filtration system be halted to clean, remove or backflush the filters.
It is therefore an object of the present invention to describe a filtration system and method that overcomes the above disadvantages of conventional filtration and ultra-filtration systems.
2. Description of the Prior Art
Schnabel et al. U.S. Pat. No. 4,411,781 describes an apparatus for ultra-filtration at pressures up to 100 bars by using suitably pressure tight diaphragms, that is membranes, as well as by modifying the normal ultra-filtration installations. The apparatus utilizes a pressure stable capillary diaphragm as the main filtration element and means for holding the diaphragm in the module. The filtration element is of a structure wherein the capillary diaphragms are permeated from the outside toward the inside as the crude solution flows in an axial direction therealong under a flow pressure gradient. The preferred diaphragms are porous glass capillaries with an external diameter of 200 to 500 micrometers with a pore size in the range of 11 to 1000 Angstroms. This reference states specifically that care must be taken to ensure the inlet flow of the crude solution into the module is not directed transversely at the capilliaries.
Hoover et al. U.S. Pat. No. 4,060,488 describes an ultra-filtration device which includes a porous support having one surface coated with a membrane that comprises at least two sizes of particles, one size capable of passing through the pores and another size capable of passing into but not through the pores. This device is said to provide an improved ultra-filtration membrane which not only provides good performance as an ultra-filtration membrane but can be regenerated or replaced while the device is installed and resists deterioration by corrosive feed stocks. The porous support has pores from about 0.5 to 45 microns in their smallest cross-sectional dimension and a particulate membrane coated onto at least a portion of the surface of the support having large inorganic particles from about 0.5 to 45 microns in average cross-sectional dimension: particles that will enter but not pass through the pores of the support and other small inorganic particles from about 0.002 to 0.5 microns in average cross-sectional dimension and of a size that will pass through the pores of the support.
Shorr U.S. Pat. No. 4,276,176 describes an apparatus for low pressure, high flux water purification. The ultra-filtration unit has at least one porous filter tube lined with a skinned ultra-filtration membrane. This particular apparatus preferably operates on impurities that are heavy metal ions and other contaminates present in waste water derived from metal finishing and plating operations. It is staeed that the fluid flow adjacent to the membrane is preferably turbulent with the pressure between 9 and 150 psig. The ultra-filtration membrane comprises a layer of open porous sponge and an extremely thin porous skin having fine pores preferably of a size approximately 10 angstroms in diameter. It is stated that in the ideal system as the particle/water blend passes through the tube under pressure the water is forced through the ultra-filtration membrane and porous tube walls. However, all particles larger than about 10 angstroms cannot pass through the pores and are carried axially along by the flow of the particle water blend in the tube.
The above references all contain conventional filter means in that the pores in the filter are of a diameter smaller than the particles that are to be excluded from passage.